This invention relates generally to the field of arc fault detectors, and more particularly to an arc fault detector with two arc fault detection levels for detecting both series and parallel arc faults.
Underwriters Laboratories requirements for certain types of arc fault circuit interrupters (AFCI), namely combination devices, as defined in their standard 1699, requires the device to sense a wide range of arcing conditions, so as to interrupt the flow of electrical current before the arcing condition causes flammable ignition of nearby combustibles. Included in these conditions are what have commonly been termed xe2x80x9cA-typexe2x80x9d faults in which the arc occurs across a break in the line or neutral conductors or at a loose terminal in a branch circuit of a distribution network, where the conductors are carrying current to a load. The arc could likewise occur as a break or at a loose terminal associated with an extension cord deriving power from line voltage, completing the circuit to the load. Since the current through the xe2x80x9cA-typexe2x80x9d fault is limited by the impedance of the load itself, because the fault is in series with the load, an A-type fault is also known as a series fault. The combination outlet device must detect series faults in which the load is as low as 5 amperes, which has been determined by UL to be the lowest current where fire is likely to occur. The highest series arc fault current slightly exceeds the rating of the upstream overcurrent device (fuse or circuit breaker) required by code to protect the branch circuit, established by UL to be 30 amperes.
xe2x80x9cB-typexe2x80x9d faults are a second arcing condition that must be detected and interrupted by a combination outlet device. In a B-type fault, the arc occurs across two conductors in the branch circuit or extension cords plugged into it at a site where the insulating media separating the two conductors has been compromised. The arc may occur across the line and neutral conductors or the line and ground conductors, or in the case of reverse polarity where the line voltage is reverse-polarized, between the neutral and ground conductors. The current through the B-type fault is not limited by the impedance of the load, but by the available current from the supply established by the impedance of the conductors and terminals between the source of line voltage and the position of the parallel fault, i.e., by the conductive members carrying the fault current. Since B-type faults are effectively across the line, they are also known as parallel faults. The combination arc fault circuit interrupter outlet device must detect parallel arc faults in which the available current to the parallel fault is as high as 500 amperes, above which the overcurrent device (fuse or circuit breaker) affords protection.
Considering both series and parallel arc faults, the combination outlet AFCI must be able to cover a range of fault currents from 5 amperes to 500 amperes. The test methodologies in the UL standard for generating series and parallel arc faults differ in order to establish the two different types of faults. The standard allows for a longer interrupting time of the AFCI for lower energy arcs (series) than for higher energy arcs (parallel) without sacrificing the protective benefit or risking ignition of nearby combustibles.
Briefly stated, an arc fault protection device uses two detection levels for detecting both parallel and series arc faults. When a sensed load current is below a predetermined level, a first set of characteristics indicative of a series arc fault are compared to a sensed di/dt signal. When the sensed load current exceeds the predetermined level, a second set of characteristics indicative of a parallel arc fault are compared to the sensed di/dt signal. This approach allows more time to evaluate a potential series arc fault than is permitted when evaluating a potential parallel arc fault.
According to an embodiment of the invention, an arc fault protection device protective of an electrical distribution system having a load includes a sensor for sensing arc fault signatures in the presence of unwanted arc fault mimicking noise; a load current threshold detector, and first and second filters for distinguishing arc fault signatures from the unwanted arc mimicking noise having first and second sets of characteristics, respectively; wherein the first filter is enabled if a load current is below the load current threshold, and the second filter is enabled if the load current is above the load current threshold.
According to an embodiment of the invention, an arc fault protection device protective of an electrical distribution system having a load includes a sensor for sensing arc fault signatures in the presence of unwanted arc fault mimicking noise; a load current threshold detector; and first and second filters for distinguishing arc fault signatures from the unwanted arc mimicking noise having first and second sets of characteristics, respectively; wherein the first filter is enabled if a load current is below the load current threshold, and the first and second filters are enabled if the load current is above the load current threshold.
According to an embodiment of the invention, an arc fault protection device protective of an electrical distribution system having a load includes a sensor for sensing arc fault signatures in the presence of unwanted arc fault mimicking noise; a first load current threshold detector; a second load current threshold detector; and first and second filters for distinguishing arc fault signatures from the unwanted arc mimicking noise having first and second sets of characteristics, respectively; wherein the first filter is enabled if a load current exceeds the first load current threshold, and the second filter is enabled if the load current exceeds the second load current threshold.
According to an embodiment of the invention, an arc fault protection device protective of an electrical distribution system having a load includes means for sensing arc fault signatures in the presence of unwanted arc fault mimicking noise; a load current threshold detector; and first and second filter means for distinguishing arc fault signatures from the unwanted arc mimicking noise having first and second sets of characteristics, respectively; wherein the first filter means is enabled if a load current is below the load current threshold, and the second filter means is enabled if the load current is above the load current threshold.
According to an embodiment of the invention, an arc fault protection device protective of an electrical distribution system having a load includes means for sensing arc fault signatures in the presence of unwanted arc fault mimicking noise; a load current threshold detector, and first and second filter means for distinguishing arc fault signatures from the unwanted arc mimicking noise having first and second sets of characteristics, respectively; wherein the first filter means is enabled if a load current is below the load current threshold, and the first and second filter means are enabled if the load current is above the load current threshold.
According to an embodiment of the invention, an arc fault protection device protective of an electrical distribution system having a load includes means for sensing arc fault signatures in the presence of unwanted arc fault mimicking noise; a first load current threshold detector, a second load current threshold detector; and first and second filter means for distinguishing arc fault signatures from the unwanted arc mimicking noise having first and second sets of characteristics, respectively; wherein the first filter means is enabled if a load current exceeds the first load current threshold, and the second filter means is enabled if the load current exceeds the second load current threshold.
According to an embodiment of the invention, an arc fault protection device, protective of an electrical distribution system which includes at least first and second conductors and having a load connected across the first and second conductors, includes detecting means for detecting a di/dt signal on one of the first and second conductors; sensing means for sensing a load current of the electrical distribution system; and determining means for determining whether the di/dt signal is indicative of noise, a parallel arc fault, or a series arc fault.
According to an embodiment of the invention, a method for protecting an electrical distribution system which includes at least first and second conductors and having a load connected across the first and second conductors, includes (a) detecting a di/dt signal on one of the first and second conductors; (b) sensing a load current of the electrical distribution system; and (c) determining whether the di/dt signal is indicative of noise, a parallel arc fault, or a series arc fault.